encoding/openapi: add test cases for non-empty lists

An upcoming change to the CUE API to make lists consistent with structs
for concreteness exposed a reliance of encoding/openapi on the current
inconsistency. It also showed that despite there being logic in
encoding/openapi for dealing with non-empty lists, that logic isn't
tested anyway.

Add a couple of test cases so that the behavior change in the upcoming
CL will be clear.

Note that the results here are not actually valid in OpenAPI 3.0, which
does not support the form of `items` generated here (see #3480).

For #3481

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Ib477e11388715482d48c8101d67a47c8efc69f60
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202243
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

Author: rogpeppe

encoding/openapi/testdata/array.cue

@@ -7,6 +7,9 @@ import "list"
 	baz?: list.UniqueItems()
 
 	qux?: list.MinItems(1) & list.MaxItems(3)
+
+	closedConcreteList?: [1, 2, 3]
+	openConcreteList?: [1, 2, 3, ...]
 }
 
 #Arrays: {

encoding/openapi/testdata/array.json

@@ -30,6 +30,71 @@
                   "type": "array",
                   "minItems": 1,
                   "maxItems": 3
+               },
+               "closedConcreteList": {
+                  "type": "array",
+                  "items": [
+                     {
+                        "type": "integer",
+                        "enum": [
+                           1
+                        ]
+                     },
+                     {
+                        "type": "integer",
+                        "enum": [
+                           2
+                        ]
+                     },
+                     {
+                        "type": "integer",
+                        "enum": [
+                           3
+                        ]
+                     }
+                  ],
+                  "enum": [
+                     [
+                        1,
+                        2,
+                        3
+                     ]
+                  ],
+                  "default": [
+                     1,
+                     2,
+                     3
+                  ]
+               },
+               "openConcreteList": {
+                  "type": "array",
+                  "minItems": 3,
+                  "items": [
+                     {
+                        "type": "integer",
+                        "enum": [
+                           1
+                        ]
+                     },
+                     {
+                        "type": "integer",
+                        "enum": [
+                           2
+                        ]
+                     },
+                     {
+                        "type": "integer",
+                        "enum": [
+                           3
+                        ]
+                     }
+                  ],
+                  "default": [
+                     1,
+                     2,
+                     3
+                  ],
+                  "additionalItems": {}
                }
             }
          },